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AC seems to run a long time

The house has a Payne HVAC system (high efficiency) that was installed in 2003, by the previous owner.

I did some tests, and my temperature difference between the supply and return ducts are aprox 15 degrees. (I know this could/should be a tad better).

I have a programmable thermostat, that I have set to stay at 80 degrees when I am not home during work hours. At aprox 4:30 it drops to 76.

The unit at times, runs for a few hours constantly. The temp inside the house stays at 78 degrees for the longest time. It almost seems it has trouble getting it below 78 degrees until the sun starts going down. (been like this ever since I bought the house in late 03).

Usually I notice this when the outside temp is in the high 80s-low 90s.

Is it normal for a system to run so long? I'm just wondering if I am wasting a ton of money on electricity by it running for hours on end. I imagine it will never turn off if I had it set to say 74 or God forbid 72.

Maybe leaving the house at 80 during the day is not a good idea, since the unit has to run so long to get it back down to temp?

Yearly maintenance is important to help maintain the efficiency, proper operation and to help reduce the risk of breakdowns of all heating and cooling equipment. If you haven't selected a service company to perform these seasonal checks since buying the home maybe now would be a good time. Have them out to check everything and give you suggestions about the best way to operate the specific system that you have. Before a breakdown is always the best time to establish a relationship with a service company.

If you live in an area with high humidity you may be experiencing longer run times than what you expect (coming out of setback) due to the equipment having to dehumidify the home before it can start reducing the temperature. The speculations (which would fill a book) on why it's doing what it is doing could range from it's operating fine to your equipment has a serious problem. Only a qualified tech at your home checking everything over is going to be able to tell you for sure what is really going on and what you can expect from the equipment you have.

Use the biggest hammer you like, pounding a square peg into a round hole does not equal a proper fit.

But hat you are describing is common.
It does sometimes take a long time to notice a temp drop.
Most likely. When you take notice to the temp not dropping below 78 for a while. That is when your system is using more of its moisture removing ability to remove moisture. Which does tend to slow down the temp drop.

@ 5 oclock it's probably about the hottest it will get for the day. Everything is heat soaked especially surfaces that get direct sun all day. Also think about all the items in the home I (mass) that get to 80* when your not home. I bet it runs for a very long time then shuts off for a few minutes then comes right back on and that time frame between each cycle will get longer and longer as you remove the heat from the mass of your home. Not to say your gaining or losing money by setting your system back. HVAC system should run constantly when at these hot temps that we have seen in the southeast. They aren't sized for full loads because they would not perform well and cost you more money at part load conditions.

Do you notice a temp difference like @ 96* it can get 76* and @ 97* it can only stay @ 77*?

Are you doing the normal lights off, shades/curtains pulled to, windows/doors/flu's shut or pulled to. Running the dryer during the hot parts of the day without makeup air can bring in a good amount of heat. Appliances put off a decent amount of heat also sometimes even in standby mode. My TV and Stereo Receiver both do.